In modern paper finishing lines, the web conventionally is passed under the guidance of different kinds of rolls through coater stations and dryer sections. Upon a web break or at the startup of the equipment, a narrow edge strip is slit from the web and then the strip is blown into a nip formed by ropes running beside the web, whereby the strip will be threaded through the coating line in the rope carrier nip. During running, the web is subjected to a high stress particularly at a coater station where the situation is further complicated by the increasing moisture content of the web. As a thin base web caliper is today preferred and the goal is to increase the use of recycle fiber as the paper raw material, the web is sensitive to high stress loads. The low strength of the web easily results in web breaks, whereby the situation is termed as critical runnability of the base web. The fragility of the web requires extremely good control of web tension and speed differentials, whereby the implementation of the control and adjustment system of the paper machine section becomes complex and the running of the equipment requires careful operation to achieve top efficiency.
As the fastest paper machines designed for coated grades are run in the production of light-weight printing grades obviously having a low-weight base web, the risk of web breaks is highest particularly in fast machines where web breaks obviously have greatest impact on profitability. Today, the fastest paper machines are run at web speeds of 1200-1500 m/min. Then, the on-machine coating line must cope with the web speed of the paper machine, and additionally, the coating line must provide a reliability figure of at least the same order as that of the paper machine. In an off-line machine, the web speed must be 10-15% higher than the maximum speed of the paper machine to prevent the coating line from forming the bottleneck at the mill. As the base paper sheet may have a basis weight of as small as 35-55 g/m.sup.2, running the moist web exiting the coater without web breaks becomes extremely difficult at these speeds.
During a web break the web tail must always be threaded through the entire paper machine section, and only after a successful tail threading, can the web be extended to its normal width. Tail threading occurs in such a manner that a narrow edge strip called the tail is slit from the web edge and guided by means of air jets into a nip formed by ropes running beside the web serving to thread the web tail through the line, after which the web is extended to its full running width by moving the edge strip slitter across the web. Guiding the edge strip into the rope nip is extremely difficult as the tail is subjected to a very high resistance by still-standing air at these machine speeds. Because the edge strip in practice has no stiffness, controlling it into the nip against the resistance of ambient air is cumbersome and requires precise support using the guiding air jets. The standard practice of tail threading at full web speed in contemporary machines occurs by blowing the edge strip into the rope nip and repeating the tail threading operation as many times as is required to successfully complete the blowing step into the rope nip and the tail threading step. As up to several tens of such attempts may be needed for each web break, it is obvious that during a web break substantial amounts of broke must be returned to the pulper and the duration of the web break is prolonged, whereby both of these shortcomings essentially reduce the operating efficiency of the line. Naturally, the advantage offered by a higher machine speed remains smaller than expected if the number of web breaks is high and the duration of the breaks is long.
It is thus evident that contemporary threading arrangements cannot be used any more if the machine speed is essentially elevated from current speeds, that is, to the very-high-speed range. The lower limit for the very-high-speed range is taken as 1800 m/min, while the design target is set as high as 2500 m/min. Obviously, the moist web exiting from a coater cannot be run in current machines at such high speeds in any case. As the air resistance increases in proportion to the second power of speed, the conventional method of tail threading will not be possible in practice. To achieve high efficiency at the high-speed range, the number of web breaks should be kept to the minimum. Also the tail threading step should go essentially smoother than today in order to keep web break downtimes and the amount of broke at a reasonable level. A rapidly moving web invokes an air flow travelling along with the web surface resulting in the entry of the air flow between the guide and pull roll, whereby disturbance and quality impairment will occur at the coater if the air is allowed to gain access between the backing roll and the web or to the application zone. Such problems are heavily accentuated with higher web speeds and concomitant increase of air resistance. The air flow induces oscillation of the web and thus increases the risk of web breaks.
As the strength of the web against stress is weakest immediately after the coat application step when the moisture content of the web has increased, attempts have been made to reduce the stresses imposed on the web by means of noncontacting web guidance. In this arrangement the web is passed between air jet cushions blown against the web from the opposite sides of the web, and the travelling direction of the web is altered by means of deflectors adapted to blow an air cushion between the web and the deflector. However, such a noncontacting arrangement is presently still hampered by several drawbacks. Namely, the web tension in this arrangement must in any case be controlled by the speed differentials of the pull rolls. Consequently, web tension control and smoothing of web tension variations remains as critical and clumsy as in roll-guided arrangements. In fact, variations in web tension form the main reason for web breaks. Implementation of tail threading in the air-jet guided arrangements is also difficult and the control of the web travel in the high-speed range would require an air-jet system of extreme precision. Such a web support system is therefore not a viable solution to the problems of web travel control or tail threading in high-speed paper machine applications.